1. Field of the Invention
This invention relates generally to an imaging system for generating the complete state of polarization of light reflected from a visible scene and, more particularly, to an imaging system that incorporates a compound prism assembly to separately and contemporaneously measure each of the four Stokes polarization parameters identifying the complete state of polarization of light reflected from a scene.
2. Discussion of the Related Art
There exists a need in the art to measure a two-dimensionally resolved, real-time complete polarimetric description or image of a visible scene. The ability to generate such a resolved image has many applications including military, industrial and commercial applications. These applications include the identification and classification of the scene according to various interests, such as land use, aircraft identification, atmospheric aerosol and pollutant identification and monitoring, cloud composition identification and monitoring, etc. A specific need for measuring a complete polarimetric image of a scene includes resolving an image of a water surface including breaking gravity waves.
In order to provide a complete characterization of the polarization state of light to generate the image, it is necessary to separately measure the light intensity occurring in each of the four Stokes polarization parameters. A convenient set of these polarization parameters include vertically planar polarized light, planar polarized light at 45.degree. from the vertical, circularly polarized light, and unpolarized light. Each of the separate measurements of the Stokes polarization parameters typically must be acquired from the same scene at the same.
Conventional radar systems have made use of radar return signals to provide plane polarimetric scattering information. Microwave backscatter returns from water surfaces have long been interpreted solely in terms of Bragg scattering, known in the art, from slightly rough surfaces with possible local tilting of the surface with respect to the nominal viewing angle. This theory predicts different absolute cross-sections for backscatter of vertically polarized and horizontally polarized incident microwave radiation as a function of the irradiating wave-length, the angle of incidence, the scattering surface dielectric constant, and the power spectral density of the surface roughness at the Bragg-resonant wave number. Evidence has suggested that radar backscatter from water surfaces which include breaking waves, requires the consideration of additional scattering mechanisms that are non-Bragg in nature to describe occurrences of very large scattering cross-sections and of unexpected polarization ratios in the scattered signal. Various scattering mechanisms have been suggested based on detailed analysis of both time-averaged and time-resolved scattering signals of grazing-angle data, polarization-ratio data, wind-direction data and wind-speed dependent data. Verification of these conjectures requires a direct measurement of the scattering surface geometry that microwave scatterometers are unable to provide.
To provide the direct identification and evaluation of scattering mechanisms, it has been suggested in the art to provide a polarimetric imaging optical specular event detector (OSED) that can be used to view the same scattering surface areas as the microwave scatterometer, and provide spatially and temporarily resolved polarimetric and photometric images that are synchronized with a radar record. A plane-polarimetric imaging system operating in the visible range (.lambda.=600 nm), synchronized to a plane-polarimetric and range-gated pulse-chirped radar (PCR) system operating at X-band (.lambda.=3 cm), has been used to elucidate the wave structures which are present during radar backscatter spiking and super events by visual identification of the wave structures present. This system is discussed in the article Barter, J. D. and Lee, P. H. Y., "Polarimetric Optical Imaging of Scattering Surfaces," Applied Optics, Vol. 35, No. 30, Oct. 20, 1996, pp. 6015-6027, herein incorporated by reference.
FIG. 1 shows a schematic plan view of an OSED 10 of the type disclosed in the Barter article. The PCR system is not shown, but its operation is well understood in the art. A white light illumination source (not shown) illuminates the particular scene, such as the water body, and reflected light 12 is measured by the OSED 10. The OSED 10, used in conjunction with the PCR, measures only the plane polarization parameters of the scene at the different frequencies. The PCR measures the plane polarization matrix (HH, VV, VH, HV) with phase information at each of the transmitted frequencies. The returns at each frequency are combined to yield target range information under the assumption that the target does not materially alter the phase beyond the phase inversion expected at reflection from an interface with a higher permittivity medium.
The reflected light 12 is analyzed with respect to the plane of incidence which includes the reflected ray 12 and the vertical normal to the average water surface. This analysis quantifies light polarized in the horizontal direction s(H) relative to the plane of incidence and in the vertical direction p(V) relative to the plane of incidence. To improve the polarization efficiency of the OSED 10, the light 12 is sent through an infrared (IR) filter 14 to remove the infrared wave lengths, because the usable spectral bandwidth of the polarizers in the OSED 10 is significantly less than the bandwidth of the cameras. The filtered reflected light 12 impinges on a beam splitter 16, such as a partially silvered mirror, to split the light 12 into a first split beam 18 and a second split beam 20 having substantially equal intensity. A beam dump 22 is provided in the unused split path to remove ghost reflections. The first split beam 18 is reflected off of a turning mirror 24 and is sent through a vertical polarizing element 26. Substantially only the vertically polarized light in the first beam 18 passes through the polarizing element 26, and is measured by a first standard charged coupled device (CCD) black and white television camera 28. The camera 28 generates an image 30 of reflections of light in the vertically polarized direction from the scene. Likewise, the second beam 20 is sent through a horizontal polarizing element 32 so that light that is substantially only polarized in the horizontal direction in the beam 12 is measured by a second CCD black and white television camera 34. The second camera 34 generates an image 36 of reflections of light in the horizontally polarized direction from the scene. Alignment control is provided by the orientation and position of the cameras 28 and 34, the orientations of the splitter 16 and the turning mirror 24 and the relative exposure and zoom settings of two camera lenses.
The measured signals from the CCD cameras 28 and 34 are applied to an image processing board 38 that adds or subtracts the images on a pixel-by-pixel basis to yield sum and difference images of the vertical and horizontal polarization of the scene. Particularly, the image processing board 38 generates a difference image 40, a sum image 42 and a degree of polarization image 44 based on the combination of the images 30 and 36. The image 44 represents a spatial distribution of the degree of polarization of the back-scattered light obtained as the pixel-by-pixel ratio of the difference and sum images.
During wave breaking events, both before and after the crest mixes turbulently with the front surface of the wave, the radar cross-section (RCS) briefly rises orders of magnitude above the ambient Bragg scattering cross-section, and is called a spiking event. During spiking events associated with wave breaking, it is also commonly found that the HH polarization exceeds the VV polarization by as much as 15 dB in contrast with Bragg scattering for which the VV polarization always equals or exceeds the HH polarization by an amount which depends on the grazing angle. This inversion of the expected Bragg scattering polarization ratio is termed a super event.
By comparison of the synchronized time records of the OSED 10 and the PCR it has been shown that the cross-section and polarization ratios measured by the OSED 10 and the PCR are well correlated so that structures identified in the visible range can be confidently identified as the sources of the spiking and super event signatures at X-band frequencies. The radar HH polarization backscatter cross-section is seen to dominate during most of the spiking event as is routinely true for mechanically generated breaking waves. Due to the choice of collected and displayed data for this run, the cross-polarized RCS returns are not available. It is known, however, from other runs, that, while the cross-polarized returns are much higher than would be expected from purely Bragg resonant targets, they are approximately balanced (VH is approximately equal to HV) and small compared to the co-polarized returns. Since the incandescent light source for the OSED 10 is not rapidly switchable between polarization states, the scene can be illuminated with unpolarized light and the OSED returns can be compared in the s-polarization and p-polarization planes to the equivalent PCR summed returns HH+VH and VV+HV, or to HH and VV as in the case where the VH and HV returns are not available.
With the demonstration of good correlation of both the cross-section and polarization ratio between the microwave and visible diagnostics, the OSED 10 provides a useful tool for the visual identification of scattering structures which give rise to the microwave spiking and super events. The OSED images have served to demonstrate the significantly s-polarized (HH) returns from the cresting but yet unbroken wave surface, the s-polarization enhancement due to Brewster reflection from the front wave surface, and the increasing dominance of p-polarization (VV) from the aging broken crest.
Measurement of the complete Stokes parameter state-of-polarization of light over a two-dimensionally resolved image would allow the description of the target in terms of a scattering matrix. Due to the wide disparity in wavelengths between the two scattering diagnostics, it is prudent to consider the extent of the disparity even though good correlation has been demonstrated. First, the difference in wavelengths corresponds directly to a difference in roughness scale length to which the radiation will be Bragg resonant. At visible wavelengths, the population of Bragg resonant features should be negligible so that the OSED response is expected not to be sensitive to Bragg-resonant features, but to be confined to optical paths which may be constructed by standard ray-tracing methods. Similarly, the regime of diffractive scattering depends upon the probing wavelength.
Estimates of the radius of curvature of scattering objects in the field of view may be obtained by considering the local image intensity in the OSED field of view. The collected power from a single scattering object with elliptical surface curvature can be written as: ##EQU1## where .PHI..sub.det is the photon power expressed in detector digitizing units (adu), collected from the scattering element and focused to a resolution limited spot on the detector, .rho. is the reflectance of the water at normal incidence, I.sub.s is the source intensity in adu/steradian, A.sub.d is the effective detector aperture, R is the effective range to the scattering element, and .kappa..sub.1 .kappa..sub.2 is the Gaussian curvature of an elliptical point on the scattering element.
An effective radius of the scattering element can then be expressed as: ##EQU2## Depending upon the size and distribution of the scattering elements in the image, and upon any blurring due either to optical resolution or target movement, the specular spot images from each scattering element may or may not be resolved. If the images are not resolved, then the effective probing depth in a multilayer array of scattering elements must also be considered. The collected power for each case is obtained from an integral of the image intensity over the scatterer image, and is estimated as: ##EQU3## where s is the relevant image size and .delta..sub.p is the effective probing depth. Using these equations, the effective radii of curvature can be estimated in the range &lt;1-10 cm for mechanically generated 4 m braking waves. In particular, the radius of curvature estimated for the unbroken crest early in the spiking event sequence is found to be in the range 3-8 cm, which is consistent with results of wave simulation codes. In the unresolved case, the estimate for r.sub.eff is to be taken as an upper limit. Various methods are described in the literature for the decomposition and analysis of the scattering matrix to allow the classification of targets according to various criteria such as odd or even bounce backscatter or target symmetries.
For the PCR, the high reflectance at each air-water interface and the short attenuation length in water will ensure that the radar returns will be dominated by scattering paths which do not travel through the water. Therefore, during the later stages of wave-breaking when the returns are provided by a broken and turbulent collection of water drops and bubbles, which may be multilayered, the PCR returns will carry information only from the first layer. On the other hand, the low reflectance and long attenuation length in water at visible wavelengths may allow significant returns from deep within a multilayered structure. It would seem probable, however, that no fundamental difference in the scattering will be achieved beyond the increase of an effectively depolarized component. However, due to the limited polarimetric data acquired (two orthogonal plane polarized components), certain assumptions become necessary for the purposes of data analysis. Firstly, it must be assumed that there is no circularly polarized component. Secondly, it must be assumed that the relative magnitudes of the horizontal and vertical components either represent a rotation of the plane of polarization with no unpolarized component, or represent the degree-of-polarization with no rotation of the incident plane of polarization.
In order to fully understand the scattering process from a water body, a complete polarimetric characterization of the scattering surface is imperative. Advanced microwave systems presently provide full plane polarization transmit-receive backscatter cross-section matrices in the horizontal and vertical polarization of the scene within the antennae footprint, but a full Stokes-parameter description of a scattering scene is not easily obtained in the microwave regime. However, a Stokes-parameter imager at optical wavelengths appears to be feasible.
Because the OSED 10 only measures the reflected light 12 in the horizontal and vertical polarized direction, the OSED 10 is limited in its ability to adequately describe the scene in terms of its polarization. For example, if the OSED 10 detects equal brightness in the vertical and horizontal polarized images 30 and 36, it cannot tell the difference between a plane polarized component that's polarized at 45.degree., a circular polarized component, or an unpolarized component. Since the OSED 10 collects insufficient information to unambiguously determine the full state of polarization, certain assumptions must be made as to the absence of circularly polarized light or obliquely oriented plane polarized light. In addition to the obvious difficulty of extending the separate camera system and TV-based signal archive method in the OSED 10 to four channels, the optical alignment of the OSED 10 is delicate and limited in the quality of image congruence by variation in the image distortions introduced by the separate zoom lens optics. The image quality is further degraded by the limited available bandwidth of the color channels and by the interlaced raster format of the TV video recording.
What is needed is an imaging device that measures a two-dimensionally resolved, real-time complete polarimetric description of a visible scene in the optical range. It is therefore an object of the present invention to provide such an imager.